CMOS image sensors are known to provide efficient image capture systems with low operating power consumption. CMOS image sensors can also be manufactured using standard integrated circuit (IC) fabrication techniques and equipment, which permits a CMOS image sensor to be easily integrated into an IC with other CMOS circuitry. Accordingly, CMOS image sensors have become the image capture system of choice in many miniature and portable systems.
FIG. 1 shows a cross-sectional view of a CMOS image sensor 100 that includes an integrated circuit die 110 containing photosensitive regions 120. Photosensitive regions 120 are arranged in a two-dimensional array with each photosensitive region 120 corresponding to a pixel in an image. Such regions 120 can be made out of positive and negative doped regions in material such as bulk silicon or amorphous silicon, or depletion regions under polysilicon or metal gates. These regions 120 behave as a capacitor when given an electrical charge, but discharge electrons with photon impingement. The rate of discharge increases proportionally to the intensity of incident light. Circuitry (not shown), for example CMOS gates, among and around photosensitive regions 120 connects to photosensitive regions 120, measures the change in charge over a known period of time for each pixel, and generates signals representing an image formed on the surface of image sensor 100.
To improve light sensitivity, image sensor 100 incorporates micro-lenses 130. Micro-lenses 130 guide light from a wider area onto underlying photosensitive regions 120. In one configuration, each micro-lens 130 corresponds to a single photosensitive region 120 and has a hemispherical shape that focuses light on the corresponding photosensitive region 120. In another configuration, each micro-lens 130 is a half cylinder overlying a row or column of photosensitive regions 120 and focuses light onto the row or column of underlying photosensitive regions 120. In either case, micro-lenses 130 require an air gap above their convex optical surfaces to properly focus incident light.
One technique for forming an array of micro-lenses 130 such as illustrated in FIG. 1 begins by coating integrated circuit die 110 with a layer of a transparent photoresist. The photoresist is then patterned to form small regions corresponding to micro-lenses 130. After patterning, heating liquefies the photoresist, and the surface tension of the liquefied photoresist causes each region to take on a convex shape that remains when the photoresist solidifies.
A cover plate over micro-lenses 130 on image sensor 100 is generally desirable to protect micro-lenses 130 from contamination and damage. However, traditional methods using an adhesive to directly attach a cover plate to image sensor 100 are not compatible with micro-lenses 130 because the adhesive that attaches the cover plate fills the required air gap above micro-lenses 130. Accordingly, image sensor 100, after being cut from a wafer, is generally placed in a housing or package having a transparent cover that protects delicate features such as micro-lenses 130. Covering micro-lenses 130 only during or after packaging can subject image sensor 100 to damage or contamination when the wafer containing the image sensor is moved from wafer processing equipment, when the wafer is cut to separate dies, and when the die is packaged. In view of the limitations of current systems for protecting image sensors, structures and methods are desired for attaching a cover to an image sensor to protect the image sensor without interfering with the required air gap above micro-lenses.
In accordance with an aspect of the invention, an image sensor has a glass plate or other transparent cover attached to a standoff that surrounds an array of micro-lenses. The standoff can be a ring of photoresist that is taller than the micro-lenses and maintains the required air gap over the micro-lenses while the transparent cover protects the micro-lenses and provides surfaces for optical coatings.
A fabrication process that attaches the cover can be performed at the wafer level using wafer-processing equipment. Accordingly, cover attachment can be performed in a clean room environment to avoid or minimize damage and contamination of the image sensor or micro-lens array before cover attachment. After attaching a plate to a wafer, the process cuts the plate to expose die pads for electrical connections. The standoff keeps the plate above the surface of the substrate, but the plate can further be grooved before attachment to the substrate to provide additional tolerance for cutting without damaging underlying circuit elements.
The application of adhesive that attaches the transparent cover to the standoff can be controlled to avoid applying adhesive to the micro-lenses. In particular, the adhesive can contain filler particles having a size approximately equal to the desired adhesive thickness to stop the adhesive from spreading onto the micro-lenses when pressure is applied during attachment of the cover. A barrier having a structure similar to the standoff can additionally or alternatively be formed between the standoff and the micro-lens array to prevent adhesive on the standoff from spreading onto the micro-lenses.
In accordance with a further aspect of the invention, the standoff (and barrier if present) can include a channel or vent that opens the air gap between the glass plate and the pixel array to the surroundings. The vent prevents thermal or external pressure changes from distorting or damaging the attached cover. The vent can be shaped to trap or prevent particles from entering and contaminating the micro-lens array.
One specific embodiment of the invention is an imaging device such as a CMOS image sensor. The imaging device includes: a substrate containing electrical elements; an array of lenses attached to the substrate; a standoff on the substrate and surrounding the array of lenses; and a transparent cover (e.g., glass plate) attached to the standoff and overlying the array of lenses. The standoff is generally taller than the lenses and made of a material such as photoresist, which is easily formed and processed using standard wafer processing equipment. The standoff can include a vent leading to a gap between the transparent cover and the array of lenses, and the vent can be shaped to permit pressure equalization but stop particles from reaching the gap and contaminating the imaging device. An adhesive attaches the cover and may contain filler particles having a size approximately equal to the adhesive thickness. An optional barrier can help stop adhesive from extending onto the lenses.
Another embodiment of the invention is a method for fabricating an imaging device such as a CMOS image sensor. The method includes: fabricating electrical components of the imaging device on a substrate; forming an array of lenses on the substrate; forming a standoff on the substrate and surrounding the array of lenses; and attaching a transparent cover to the standoff. The process can be conducted at the wafer level using standard wafer processing equipment. One process for forming the standoff deposits a layer of photoresist on the substrate, exposes selected regions of the photoresist to define the area of the standoff, and develops the photoresist to leave a portion of the photoresist from which the standoff is formed. Applying an adhesive to a top surface of the standoff and pressing the transparent cover onto the standoff attaches the transparent cover to the substrate.
In accordance with a further aspect of the invention, the adhesive used to attach the cover to the standoff contains filler particles such as glass balls having a size or diameter about equal to the desired adhesive thickness. The filler particles prevent the plate from being pressed into direct contact with the standoff and prevents the adhesive from being completely squeezed off of the standoff and onto nearby lenses.
The substrate can be a wafer processed to form multiple substantially identical integrated circuits, with the imaging device being one of the integrated circuits. On a wafer, a glass plate or other cover plate can be attached to the standoffs in all of the integrated circuits. Cutting the transparent cover removes portions of the transparent cover that overlie active circuitry in the substrate but still leaves an underlying portion of the substrate intact. The standoff provides a separation between the cover and the substrate and precut grooves on an underside of the plate can provide tolerance necessary to ensure that sawing the cover does not damage underlying circuitry. Further cutting of the wafer and the cover plate separates individual IC dies.